1. Field of the Invention
The present invention relates generally to a control method and system of a fuel cell system and, more particularly, to a control method of a fuel cell system that prevents deterioration of a fuel cell stack and diagnoses a failure in a load for draining residual voltage of the fuel cell stack.
2. Description of the Related Art
A fuel cell system applicable to a hydrogen fuel cell vehicle, a type of eco-friendly vehicle, is composed of a fuel cell stack to generate electric power from an electrochemical reaction of reactant gases; a hydrogen supplying system configured to supply hydrogen as fuel to a fuel cell stack; an air supplying system configured to supply gas including oxygen as an oxidant in electrochemical reactions; and a heat and water management system configured to manage water and maintain an optimal fuel cell stack temperature for driving by emitting heat, which is a by-product of the electrochemical reactions therein.
FIG. 1 is an exemplary schematic view illustrating an overall fuel cell system. As shown in FIG. 1, the fuel cell system 100 may include a fuel cell stack 10, a fuel cell load device 20, an air blower 30, a humidifier 40, air shutoff valves 35 and 45 in an inlet and outlet, a drain valve 42, a purge valve 44, a water trap 50, a hydrogen recirculation device 55, a hydrogen supply valve 57, a radiator 60, and a thermostat 65. The air shutoff valves 35 and 45 in the inlet and outlet may prevent inflow of an air to the fuel cell stack after a fuel cell vehicle shuts down. The drain valve 42 is disposed in a hydrogen exhaust line to remove generated water at an anode, and the purge valve 44 adjusts hydrogen concentrations at the anode and exhausts hydrogen to an air outlet to dilute the air.
The fuel cell load device 20 that lowers the voltage of the fuel cell stack 10 to drain the voltage is connected to the fuel cell stack 10 while the fuel cell vehicle is shutting down and after the fuel cell vehicle shuts down, to remove oxygen inside the fuel cell stack 10. The oxygen flowing into the fuel cell stack 10 is removed with residual hydrogen of the anode as the fuel cell load device 20 consumes current. However, insufficient hydrogen may preclude complete consumption of the oxygen, and thus a Wakeup technique is used to periodically supply hydrogen to the anode.
In other words, unlike an internal combustion vehicle, the fuel cell system 100 requires a post-processing procedure that lowers the voltage of the fuel cell stack 10 by removing residual air inside the fuel cell stack 10 after the fuel cell vehicle shuts down. The post-processing procedure prevents deterioration of the fuel cell stack 10 to avoid the danger of exposure to a high voltage. When voltage is formed as oxygen exists at the anode, carbon corrosion may occur at a cathode. Accordingly, it is necessary to remove the oxygen inside the fuel cell stack 10 and to prevent the additional inflow of oxygen. When additional oxygen flows into the fuel cell stack, draining oxygen removal is necessary. Consequently, when a vehicle shuts down, the system stops the supply of oxygen, intentionally consumes the residual oxygen for load current using the fuel cell load device 20, and decreases the voltage.